THEORETICAL REMARKS 



1253 



has a high value of A in equation (31.4). According to the theory of the activated com- 

 plex (c/. equation 31.7) a high value of A ( = ce^'^"/'^^) means a large entropy, Sa of the 

 activated state; and a reaction in which six bonds are disrupted simultaneously must 

 increase the molecular disorder, i.e., lead to an increase in entropy. (This hypothesis is 

 similar to the theory of denaturation of proteins of Stearn and Eyring.) 



Wohl analyzed the P'"'""' = f{T) curves given ty Warburg and by Emer- 

 son for Chlorella, and by Emerson and Green for Gigartina, by assuming 

 two consecutive dark reactions, both occurring at the same "reduction site" 

 {i. e., at the same enzyme molecule), one requiring the time tai and the 

 other the time ta2- Under these conditions, the dark reaction time, ta, 

 derived from flashing light experiments (chapter 34) is simply the sum of 

 the two consecutive reaction periods, ta = t,i + 1^2- Wohl found it possible 

 to reproduce the experimental data closely by assuming that the first reac- 

 tion has an A value in the Arrhenius formula characteristic of a bimolecular 

 reaction (with the reaction partner present in a concentration of 10 "^'^ to 

 10 ~^^ mole/1.) , and a low activation energy (0-9 kcal/mole), while the 

 second one is a monomolecular reaction with a very high activation energy 

 (23-58 kcal/mole), and a high activation entropy. This second reaction 

 was the one he interpreted as the liberation of a complex (Ce) molecule, by 

 simultaneous dissociation of several (six?) bonds attaching it to the en- 

 zyme. The bimolecular reaction accounts for 73-85% of the total dark 

 reaction time at 15-25° C, and for only 12-24% at 5° C. 



Wohl pointed out himself that with the four arbitrary constants avail- 

 able (two A values and two Ea values), the possibility of representing the 

 experimental curves by the theoretical equation is not in itself significant ; 

 but he considered it significant that the two calculated reactions are so dif- 

 ferent in character, and, in particular, that the reaction that seems most 

 important at low temperatures has the character of a monomolecular reac- 

 tion with an extraordinarily high activation energy. 



The attempt of Tamiya, Huzisige and Mii (1948) to analyze the temper- 

 ature curves of photosynthesis, in terms of two or three consecutive dark 

 reactions, each obeying the Arrhenius law, was mentioned in sections 2 

 and 3. The success of mathematical analyses such as those of Wohl and 

 Tamiya does not prove that the assumption on which they are based is 

 necessarily correct. 



It may be doubted whether the rapid decrease in the rate of photo- 

 synthesis at low temperatures (as well as the similar phenomenon occur- 

 ring above 30-35° C.) is at all due to a reaction that constitutes a step in 

 the "catenary series" of photosynthesis. It seems more probable that 

 these changes are due to alterations in the colloidal structure of the proto- 

 plasm, which affect, although to a different degree, all physiological proc- 

 esses taking place in the cell (as the freezing or evaporation of a solvent 



